In the past several years, it has become increasingly apparent that prokaryotic organisms often use two-component regulatory systems to communicate with their environment. These systems allow bacteria to sense their common habitats and respond appropriately. This response usually involves coordinated control of gene expression and this ability provides a selective advantage important for cell survival. The enteric bacteria, Escherichia coli, has a number of two-component regulatory systems, and several have been characterized in some detail. One example is the system that controls expression of the major outer membrane porin proteins, OmpF and OmpC, in response to media osmolarity. This system contains a transmembrane receptor that senses media osmolarity and transmits this information by phosphorylation and dephosphorylation to the regulatory protein OmpR. Under diluted conditions, concentrations of intracellular OmpR-P are maintained at low levels resulting in preferential expression of ompF. In the intestine, osmolarity is high and concentrations of OmpR-P are increased. This activates ompC, which specifies a more protective pore, and represses expression of ompF. Previous results establish a connected signal transduction pathway that extends from the periplasm of the bacteria to the alpha-subunit of RNA polymerase at the porin gene promoters. Our long term goal is to understand this information flow in molecular terms. Experiments proposed utilize genetics to identify key regions in both the receptor and the DNA-binding protein that participate in the signal transduction process. Characterization of these mutants should allow insights into the specific functions performed by these key regions. In addition, strategies are described to probe the mechanistic implications of the interaction between OmpR-P and RNA polymerase. Results obtained should be directly applicable to similar regulatory systems in pathogenic bacteria, and may suggest new approaches for the treatment of infectious disease.